This invention relates to an overflow valve for use in a distributor-type fuel injection pump.
Internal combustion engines, particularly Diesel engines with divided combustion chambers have some difficulties in starting at low temperatures. As a starting aid, a glow plug has conventionally been used, which preheats the intake air. Besides such starting aid, the timing device of the fuel injection pump is adapted to advance the fuel injection timing at the start of the engine, to thereby improve the startability of the engine as well as reduce emission of hydrocarbons from the engine. A timing device provided in a distributor-type fuel injection pump is generally designed to be controlled by fuel pressure within the suction chamber of the fuel injection pump. However, according to such a timing device, at the start of the engine, the rotational speed of the engine is so low that the suction chamber pressure is not yet increased to a level sufficient for the timing device to obtain a required advance in the fuel injection timing. That is, the suction chamber is supplied with fuel pumped by a feed pump which is driven by the engine, and therefore the suction chamber pressure varies in proportion to the rotational speed of the engine so that the timing device responsive to the suction chamber pressure eventually operates in response to the rotational speed of the engine. Therefore, it is necessary to control the fuel injection timing for improvement of the startability of the engine so as to advance same independently of the rotational speed of the engine upon and immediately after the start of the engine.
To comply with such requirement, a timing device has been proposed by Japanese Provisional Utility Model Publication No. 55-49078, which is adapted to advance the fuel injection timing at the start of an internal combustion engine. According to this proposed timing device, an auxiliary piston is arranged on an extension of the axis of the timer piston of the timing device for urging contact therewith by the force of an auxiliary spring. One end of the auxiliary piston cooperates with an opposed end of the timer piston to define a pressure chamber supplied with fuel having pressure variable as a function of the rotational speed of the engine from the suction chamber of a fuel injection pump associated with the timing device. When the pressure of fuel supplied into the pressure chamber is low at the start of the engine, the auxiliary spring urgingly biases the timer piston in the injection timing-advancing direction, to thereby obtain an advance in the injection timing. Then, as the pressure within the suction chamber increases with an increase in the engine rotational speed, the auxiliary piston is moved by correspondingly increased pressure within the pressure chamber against the force of the auxiliary spring, accompanied by movement of the timer piston in the injection timing-retarding direction by the force of a timer spring urging same toward the auxiliary piston, to retard the fuel injection timing. After this, as the engine rotational speed further rises to cause a further corresponding increase in the suction chamber pressure, correspondingly increased pressure within the pressure chamber causes movement of the auxiliary piston to its most receded position while compressing the auxiliary spring, and thereafter a further increase in the engine rotational speed causes movement of the timer piston in the injection timing-advancing direction against the force of the timer spring, whereby injection timing control is effected in response to the varying engine rotational speed.
However, the above proposed timing device is inevitably large in size and complicated in structure due to the structural disadvantage that the timer piston is directly driven by the auxiliary piston arranged in line therewith to obtain an initial starting advance in the injection timing, requiring a large mounting space and a large manufacturing cost.
As another measure for obtaining an initial starting advance in the injection timing, if the setting pressure of a pressure-regulating valve for regulating the pressure of fuel supplied into the suction chamber is set at a larger value than a usual value, an advance can be obtained in the injection timing at the start of the engine. However, according to this another measure, the timing device is kept in the injection timing-advancing position even after the starting condition of the engine is over, and then the injection timing cannot be controlled to best values appropriate to the rotational speed of the engine.